The MST containing the edges with weights x,y,z will look like:
For the maximum value of x consider this:
The vertex F can be connected to the graph by selecting either edge A-F or edge F-G. Now the weight of A-F is 11. So if x is greater than 11 then instead of selecting the edge F-G we would have selected A-F while constructing MST. So to make sure that the edge with weight 'x' is included in MST, the maximum value of x can be 11, i.e., x<=11.
For maximum value of z:
The vertex can be connected to the graph by edges A-B or B-G or B-H apart from the edge B-C having weight z. But inorder to ensure that the edge with weight 'z' is included in MST, the edge B-C should have maximum weight= the least weight among the other mentioned edges. So B-C can have weight at max=8 or else we would not have selected the edge while constructing the MST in the first place and rather have gone for edge B-H having least weight=8 in that case. So z<=8.
For maximum value of y:
Similarly 'y' can be at max=6 because say 'y'=7 or 8 or any value>6, in that case, we would not have included the edge D-E in the MST and rather have selected edge D-I as it would be the least weighted edge then.
So x<=11, y<=6, z<=8.
Hence max(x+y+z)= 11+6+8 = 25
Please correct me if I am wrong.